1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) transmission apparatus applied to a WDM optical transmission system. More particularly, the invention relates to setting of the gain of a pre-amplifier utilizing ASE (Amplified Spontaneous Emission) light in the WDM optical transmission apparatus.
2. Description of the Related Arts
Optical transmission systems employing WDM technique are in practical use. As for the WDM optical transmission apparatuses used in such optical transmission systems, in the increasing progress of the optical transmission technology in recent years, practical use of WDM ring networks using optical ADM apparatuses supporting the function for inserting/dividing (ADD/DROP) optical signals by every optical wavelength as a unit and an optical path protection switching function has been required.
FIG. 1 illustrates an example of the structure of a WDM transmission apparatus placed in each node in a network. The portion surrounded by the dotted line is an optical ADM apparatus 100 having functions for inserting/dividing (ADD/DROP) an optical signal and as an optical path protection switch.
Wavelength division multiplexed (WDM) signals are input bi-directionally on the input side and the output side of this optical ADM apparatus 100 and are input through pre-amplifiers 101, 115 to wavelength demultiplexing apparatuses 102, 114 in the optical ADM apparatus 100. The WDM signals are separated into optical signals having respective wavelengths in the wavelength demultiplexing apparatuses 102, 114.
The wavelength demultiplexed optical signals are input into an optical switch unit 103, switched to a predetermined route and output. Furthermore, at a node an optical signal is inserted (ADD) from a transponder not shown in FIG. 1 and is branched (DROP).
Now, a schematic view of the function of the optical switch unit 103 is shown in FIG. 2. In FIG. 2, the switch unit 103 is mounted with a 2×2 switch for each wavelength λ. Each 2×2 switch has two states of Through setting (FIG. 2A) and ADD/DROP setting (FIG. 2B).
In the Through setting in FIG. 2A, the output signals from the pre-amplifiers 101, 115 are switched to the input sides of post-amplifiers 105, 111 through the 2×2 switch 103-1.
In ADD/DROP setting shown in FIG. 2B, a signal on an working side W from a transponder not shown in the figure is input from an ADD terminal 125 and is switched to the input sides of the post-amplifiers 105, 111 through a 2×1 switch 103-2 and the 2×2 switch 103-1. Furthermore, when dividing a signal at this node being input from the pre-amplifiers 101, 115, the signal is switched to the working side and an auxiliary side and divided by a coupler 103-3 through the 2×2 switch 103-1.
The wavelength demultiplexed optical signal output from the optical switch 103 and the optical signal inserted through the 2×1 switch 103-2 are adjusted for their levels through Variable Optical Attenuators 106, 116 and are input to wavelength multiplexing apparatuses 104, 112. Then, the optical signals are again input to the post-amplifiers 105, 111 as WDM signals wavelength multiplexed at the apparatuses 104, 112.
In such a conventional WDM transmission apparatus shown in FIG. 1 and FIG. 2, the post-amplifiers 105, 111 are always operated in an AGC mode and the pre-amplifiers 101, 115 are operated in the AGC mode after they have been undergone with gain setting in an ALC mode.
Now, the meanings of the AGC mode and the ALC mode will be described.
The AGC (Automatic Gain Control) mode is a control mode maintaining the ratio of optical input levels into post-amplifiers 105, 111, optical amplifiers, and the optical output levels from them (i.e., gain) constant.
In the AGC mode, when an input level into an optical amplifier is varied, the output level of the optical amplifier also varies following the input level because the gain of the optical amplifier is constant. When the number of multiplexed wavelengths of an optical signal input into an optical amplifier varies, because of the constant gain, the output level of each wavelength does not vary if the input level of each wavelength does not vary. Thus, the services using the existing wavelengths is not affected if a new wavelength is added or a wavelength is deleted.
On the other hand, where the targeted output level per one (1) wave having the number of multiplexed wavelengths n is represented by Pn, the ALC (Automatic Level Control) mode is a mode for controlling the gain of an optical amplifier such that the total output Po from the optical amplifier is maintained constant (=Pn×n) being independent of the input levels into the pre-amplifiers 101, 115, optical amplifiers. Therefore, an optical amplifier needs information about the number of multiplexed wavelengths n to obtain the total output Po.
That is, in the ALC mode, an optical amplifier is controlled such that the total output level of the optical amplifier is maintained constant even when the input level into the optical amplifier varies.
Thus, when one of the wavelength multiplexed wavelengths in the optical signal input into an optical amplifier is off, the total output level is controlled by the ALC control to be maintained constant. Thus, the wavelength components except for the wavelength which is off are more strongly amplified.
As a result, because the information about the number of the multiplexed wavelengths n is updated later than the variation of the optical level at the time when a wave is made off has been updated, a data error is likely to occur due to a deviation from the targeted output level during the time period between the updates.
On the other hand, when a new wave is added to an optical signal to be input into an optical amplifier, the optical amplifier output level of the wavelengths except for the added wavelength is lowered until the number of the multiplexed wavelengths n is updated because the total output level is maintained constant by the ALC control.
As described above, in the ALC mode, the output level of each wavelength varies when the number of wavelengths input into an optical amplifier is increased or decreased and an error occurs in the signal when the output level deviates from the tolerance of the input level of an O/S (optical/Signal) conversion module receiving this light with the wavelengths.
Therefore, the mode is switched into the AGC mode after a gain setting has been completed in ALC mode in advance in order to maintain the targeted output level constant if an increase or decrease of the number of the wavelengths occurs.
The sequence of a pre-amplifier gain setting in such a conventional apparatus is shown in FIG. 3. Then, the relation between a node 1 and a node 2 connected to each other through an optical transmission path 122 as shown in FIG. 4 will be described.
The number of the wavelengths is notified by a control unit not shown in FIG. 1 to the post-amplifier 105 at the node 1 as an initial setting (Process 1). Then, as described later, the number of the wavelengths is notified by an OSC (Optical Supervisory Channel) signal one after another up to the pre-amplifier 201 at the next node 2 because the number of the wavelengths is necessary for gain setting at a pre-amplifier 201 at the next node 2 (Process P1-1, P1-2, P1-3).
The post-amplifier 105 always operates in the AGC mode when its input level exceeds the threshold value while it is being activated (Process P2) and its output level becomes stable when Variable Optical Attenuator (VOA) 106 is adjusted by the control of variable optical attenuation controller 107. When the output level becomes stable, output stability information is generated (Process P3). This output stability information is sent to an OSC unit, described later, at the node 1 (Process P2-1). Then, as shown in FIG. 4, the output stability information is superimposed on the OSC signal and is sent to the OSC unit at the next node 2 through the optical transmission path 122 (Process P2-2).
Now, in order to stabilize the output level of the post-amplifier 105, it is necessary for a through light (see FIG. 2A) to be stable without any variation of the number of wavelengths and it is necessary for an inserted (ADD) light to be stable without any variation of the number of wavelengths and necessary that the output light from the transponder is stable.
Therefore, the post-amplifier 105 supervises the output level and sends post-amplifier output level stabilizing bits to the pre-amplifier 201 at the next node 2 by the OSC signal as output stability information. At the node 2, such output stability information is obtained from the OSC signal and sent to the pre-amplifier 201 at the node 2 (Process P3-3). At the pre-amplifier 201 at the node 2, when it is known by the wavelength information from the post amplifier 104 at the previous node 1 that the number of the wavelengths is one (1) or more and, the output stability information is received, a gain setting is conducted in the ALC mode (Process P4). When the gain setting is completed, the mode of the pre-amplifier 201 shifts to the AGC mode (Process P5).
In this way, in the conventional WDM transmission apparatus, the post-amplifiers 105, 111 are always in the AGC mode and the pre-amplifiers 101, 115 can not conduct any gain setting in the ALC mode without an optical input corresponding to the level of one (1) or more wavelengths.
Therefore, as a system, it is possible to conduct a gain setting for the pre-amplifiers 101, 115 over a span (space between nodes) where an optical level of one (1) or more wavelengths exists.
However, when the working line is switched to the auxiliary line because of occurrence of a failure or when the network is re-constructed, the span becomes zero (0) wavelength span and the optical level becomes lower than that of one (1) level. As a result, the input level of the post-amplifiers vary because of the variation of the number of the wavelengths and the pre-amplifier at the previous node being conducted a gain setting. In such a case, it is impossible to conduct any gain setting. In terms of this, in the conventional system, this process can not be called an activation that is completely span-independent.
In this way, in the conventional WDM transmission apparatus, in such cases as where activating the apparatus after recovering from a failure or where newly activating a system constructed in a network, it is possible to conduct a gain setting of the pre-amplifier at the next node only when the conditions that the optical level input into the post-amplifier in the span exist exceeding that of one (1) wavelength and that the output is stable are satisfied.
That is, in the pre-amplifier, the gain setting is not conducted when the post-amplifier output level stabilizing bits does not indicate the stability of the pre-amplifier. In other words, the gain setting of the pre-amplifier of the next node can not be started when the optical level input into the post-amplifier does not become stable.
Likewise, in a ring network in which a plurality of nodes are connected in a ring, when gain setting of pre-amplifiers in all the nodes are conducted at the same time, as to the gain setting of each span, the gain setting can not be conducted when the gain setting for the previous span has not yet been completed if there a through light from the previous node. Therefore, as a result, each gain setting is completed one after another from the node where a wavelength λ is inserted (ADDED). An example of a ring network is shown in FIG. 5.
In FIG. 5, nodes 1 to 6 are structured to connect as a ring. When a wavelength λ 1 is inserted (ADDED) from the node 1 and passes through from the node 2 to the node 5 and is divided (DROP) at the node 6, if the gain setting for the node 1 to 6 are started at the same time, the pre-amplifier 201 at the node 2 waits for the output stabilizing bits from the post-amplifier 105 at the node 1 and the post-amplifier 205 at the node 2 can not send the output stabilizing bits because the optical output level from the pre-amplifier 201 is less than one (1) wave level or is not stable.
Likewise in turn, at the node 3 to the node 6, pre-amplifiers 302 to 602 and post-amplifiers 305 to 605 are subject to the stabilization of the output of the post-amplifier at the previous node.
Gain setting triggers of the pre-amplifiers 102 to 602 conduct the gain setting after the pre-amplifiers are mounted on a shelf and in-service setting is completed when the pre-amplifiers automatically monitor the output stabilizing bits from the post-amplifiers in each of the previous node and confirms that the bits are stable. In other words, the post-amplifier at the previous node does not have a structure in which when the pre-amplifier in the next stage conducts the gain setting is not recognized.
Summarizing above, in the conventional apparatus, when a pre-amplifier conducts a gain setting in the ALC mode, the input level and the output level of the pre-amplifier are monitored and the difference between them are obtained as a gain. However, because conducting the gain setting is not possible when the input level to the pre-amplifier is less than that of one (1) wave, there is a problem that conducting a gain setting of a pre-amplifier is impossible at zero (0)-wave span.
If conducting the gain setting of pre-amplifier is impossible at a zero (0)-wave span, there is a problem that, when an OSPPR (Optical Shared Path Protection RING) service channel (for example, the line constituted of nodes N1 to N5 in FIG. 6A) is switched to a protection path (the line constituted of the node N1, N8, N7, N6 and N5 in FIG. 6A) as shown in FIG. 6, the requirement of the protection switching time (within 50 ms) can not be satisfied because the gain setting of amplifiers in the path is started at the time of the above switching.
In order to solve the above problems, a structure has been suggested in which a light containing one or more waves always exist in each span in the RING by holding an OUPSR (Optical Unidirectional Path Switched RING) of one (1) or more wave in the RING or by bridging ADD light of OSPPR (Optical Shared Path Protection Ring) shown in FIG. 6B and, therefore, passing a light through the protection path. However, the above suggestion leaves a problem that the structure must impose some restrictions for system operation on the customers.
It is also considered to provide a light source in the post-amplifier unit and to use this light source when conducting the gain setting of the pre-amplifiers in zero (0)-wave spans. However, this leaves a problem that it results in an increased mounting area on the unit and an increased cost of the light source and the switching device.